show the header if both this and dialog background fallback

to be correctly disaplayed the header needs to come from te same theme
as the dialog background.
but if both the header and background do fallback, it is safe to use it

src/declarativeimports/plasmaextracomponents/qml/PlasmoidHeading.qml

@@ -63,9 +63,16 @@ T.ToolBar {
 
     background: PlasmaCore.FrameSvgItem {
         id: headingSvg
-        visible: fromCurrentTheme
+        // This graphics has to back with the dialog background, so it can be used if:
+        // * both this and the dialog background are from the current theme
+        // * both this and the dialog background are from fallback
+        visible: fromCurrentTheme === backgroundSvg.fromCurrentTheme
         imagePath: "widgets/plasmoidheading"
         prefix: position === T.ToolBar.Header ? "header" : "footer"
+        PlasmaCore.Svg {
+            id: backgroundSvg
+            imagePath: "dialogs/background"
+        }
 
         colorGroup: control.PlasmaCore.ColorScope.colorGroup
         PlasmaCore.ColorScope.inherit: false